As indicated, for example, in U.S. Pat. No. 4,287,512, granted Sept. 1, 1981, and entitled "Magnetic Locking Methods And Apparatus", magnetic door locking arrangements have been proposed heretofore, and as one aspect of these systems it is desirable to determine whether or not the door is open or closed, and correspondingly, whether or not the magnetic strike plate is in engagement with the electromagnet, or is properly coupled with the electromagnet rendering the door properly locked. In U.S. Pat. No. 4,287,512, this is accomplished through the use of a Hall effect element which detects the level of the magnetic flux in the electromagnetic core.
The approach disclosed in U.S. Pat. No. 4,287,512, however, suffers from one major and serious drawback: its complexity. An electromagnetic lock is fundamentally a simple device. It basically consists of a coil and a magnetic core normally potted within a casing, and a strike plate. Such a devie is rugged, reliable, inexpensive, and capable of being operated by a wide range of unregulated voltages. When a Hall effect generator is introduced into such a device, several problems result. First, the Hall effect generator requires a regulated voltage, which is not a requirement of the magnet itself and therefore introduces significant additional costs. Second, the output of the Hall generator is a varying weak low-voltage signal, which must be amplified before it can be put to use. The Hall generator chips themselves must, therefore, be accompanied by a large number of additional components to function properly. Any of these components may fail, and such failure will require taking the lock out of service, which will present the owners with repair problems. Moreover, the structure as shown in the above patent envisions the Hall generator as being within the electromagnet and the associated driving and amplifying circuitry being housed in a separate box. The cost of purchase and installation of the separate box is a strong negative relative to the approach disclosed in the above patent. Accordingly, although the electronics box itself is repairable, the Hall generators are typically potted within the lock and as such are unrepairable.
An additional disadvantage of the Hall generator approach is that its final output is a voltage signal. Most users of magnetic locks in buildings prefer an isolated contact closure signal, because this corresponds to existing practice in determining the status of doors. Commonly, within commercial buildings and many homes, magnetic contact switches are placed on each door, such that if the door is closed, the switch will be closed; and the switches may be connected in series. Some type of central panel, then, typically monitors all the doors by putting out a low current signal. If the signal returns to the panel, all the switches which are wired in series must be closed. If the signal does not return, one of the doors is open. It is, therefore, desirable that door status sensing employed with magnetic locks be available in contact closure form, so that it will be compatible with existing facility control panels. To obtain such a contact closure signl from a Hall effect device, an electromechanical relay must be added, following amplification, and this introduces further cost and unreliability.
Other prior art alternatives include the use of snap-action microswitches embedded in the face of the magnet so that, when the strike plate closes against the magnet, the switch is activated; and magnetically activated reed switches embedded in the magnet to be activated by a permanent magnet embedded in the strike plate. These techniques have two main drawbacks. First, and most seriously, they are relatively insensitive. More specifically, they will respond to the presence of the strike plate near the magnet face even if a substantial air gap is present. In the case of the snap-acting microswitch, a substantial amount of non-permeable material may introduce an air gap between the magnet and the strike plate, and the switch will still be activated. Similarly, in the case of the reed switches, they typically trigger when the strike plate approaches within one-quarter inch of the magnet face. Attempts to position the reed switch farther from the corresponding magnet in the strike plate in order to increase its sensitivity will cause it to function erratically or not at all. Reed switches are, by their nature, incapable of precise action. These deficiencies mean that a person could defeat the action of the electromagnet and strike plate by inserting a relatively thin layer of cardboard or the like so that the door could still be opened at will, while an indication of full closure would be sensed.
A second significant limitation on this type of switch is that reed and snap switches, being electromechanical devices, often fail and require replacement, particularly when they are subjected to shock; and this is a frequent occurrence in the use of a magnetic door lock since the strike plate is intended to slap against the magnet face. Although the reliability of snap-acting switches and reed switches are adequate for many applications, their use in magnetic locks seriously degrades the utility of the magnetic locks. More specifically, magnetic locks by their nature possess no moving parts and have theoretically unlimited lives, and are mounted in a strong and permanent fashion so as to resist tampering by vandals. The presence of internal components such as snap-acting switches or reed relays introduces parts which may easily fail or go out of adjustment, and thus significantly reduces the utility and value of the electromagnetic locking systems.
Accordingly, a principal object of the present invention is to overcome the problems and difficulties encountered heretofore with magnetic lock status detection arrangements such as those outlined hereinabove, and, more particularly, to provide a simple, economical and reliable magnetic lock status detector which is not an electronically sensitive component and which does not have any moving parts.